CMOS devices with very thin gate dielectrics made from silicon dioxide may experience unacceptable gate leakage currents. Forming the gate dielectric from certain high-k dielectric materials, instead of silicon dioxide, can reduce gate leakage. Because, however, such a dielectric may not be compatible with polysilicon, it has been suggested that metal gate electrodes replace polysilicon based gate electrodes in devices that include high-k gate dielectrics.
Although metal gate electrodes may be used to form both NMOS and PMOS transistors, it may not be possible to generate gate electrodes with optimal workfunctions, if the same material is used to make metal gate electrodes for both types of transistors. It may be possible to address this problem by forming the NMOS transistor's metal gate electrode from a first material and the PMOS transistor's metal gate electrode from a second material. The first material may ensure an acceptable workfunction for the NMOS gate electrode, while the second material may ensure an acceptable workfunction for the PMOS gate electrode. Processes for forming such dual metal gate devices may, however, be complex and expensive.
A PMOS transistor with a silicide gate electrode may manifest acceptable drive current and mobility properties, even when its gate dielectric is made from a very thin layer of silicon dioxide. NMOS transistors with silicide gate electrodes and very thin silicon dioxide gate dielectrics may not, however, have similar characteristics.
Accordingly, there is a need for a semiconductor device that includes both NMOS and PMOS transistors, in which the PMOS transistor comprises a silicide PMOS electrode that is formed on a very thin gate dielectric layer. There is a need for a relatively inexpensive and uncomplicated process for making such a CMOS device that shows acceptable transistor performance for both types of transistors. The present invention provides such a semiconductor device and a method for making it.